Amyotrophic lateral sclerosis (ALS) disease was first described by the French neurologist to Jean-Martin Charcot and its name reflects both the degeneration of corticospinal motor neurons, the descending axons of which show altered structure in the lateral spinal cord (lateral sclerosis) and the demise of spinal motor neurons, with secondary denervation associated with muscle wasting (amyotrophy) (Taylor, Brown, and Cleveland 2016). Indeed, ALS is a progressive and ultimately fatal neurodegenerative disease resulting from motor neuron degeneration in the cerebral motor cortex, the brainstem and spinal cord involved in the planning, control and execution of voluntary movements. Fatal outcome typically occurs 3-5 years after diagnosis (Taylor, Brown, and Cleveland 2016). The prevalence of ALS approximately reaches 5 cases in 100 000, which reflects the rapid lethality of the disease (Taylor, Brown, and Cleveland 2016). About 10% of ALS cases appear to be genetically transmitted in families (hereditary ALS), in association with specific genomic mutations. For example, approximately 20%/o of familial ALS is associated with a mutation in the superoxide dismutase (sod1) gene (Vucic and Kiernan 2009; Rosen 1993). Other non-familial cases are classified as sporadic ALS (90% of ALS cases) (Lagier-Tourenne and Cleveland 2009), meaning that it occurs without a family history.
Neurodegenerative disorders, such as Parkinson's, Huntington's, Alzheimer's disease, frontotemporal lobar degeneration (FTLD) and ALS are associated with the accumulation of misfolded proteins both inside and outside of neuronal and glial cells in the central nervous system (Polymenidou and Cleveland 2011). These misfolded protein aggregates are pathological hallmarks of each disease and can spread from cell to cell through a prion-like mechanism after an initiating event. One widely held view is that these aggregates play a critical role in disease initiation and progression, with the misfolded versions of endogenous proteins likely to acquire toxic properties, potentially through increased hydrophobicity and/or sequestration of essential cellular components within the aggregates, generation of oxidative species, proteasome inhibition and through other pathways. An alternative view is that the large aggregates do not represent the toxic form, but the final product of a defensive cell response aimed at protecting cells from more toxic oligomeric species that remain undetectable by most techniques (Polymenidou and Cleveland 2011).
Studies of serum samples from patients with ALS, but seronegative for Human Immunodeficiency Virus (HIV) or Human T cell leukemia virus (HTLV) exogenous viruses, showed reverse transcriptase (RT) activity in 50-60% of ALS samples with level comparable to those of HIV-infected patients (MacGowan et al. 2007; McCormick et al. 2008; Andrews et al. 2000; Steele et al. 2005). This is consistent with the fact that retroviral involvement has been suspected for several years since the recognition that both murine and human retroviruses can cause ALS-like syndromes (McCormick et al. 2008). ALS-like disorder in HIV-positive patients can remit with antiretroviral therapy (Moulignier et al. 2001; von Giesen et al. 2002). This is valid for ALS symptomatology in HIV-infected patients and may nonetheless represent a peculiar sub-category of ALS cases.
Increased RT activity was also found in serum of ALS patient's first degree relatives, which leads to the speculation that RT activity may derive from inherited active copies among human endogenous retroviruses (HERVs), which represent 8% of our genome (Steele et al. 2005).
Nonetheless, the detection of RT activity as such in ALS does not identify the origin of this enzyme, but the involvement of HERV-K in post-monrtem brain, (Douville et al. 2011)] has been shown. Sequencing studies revealed that the 7q34 and 7q36.1 chromosomal loci (corresponding to HML-2 and HML-3 subfamilies of HERV-K respectively) are more frequently expressed in patients with ALS, compared to controls (Douville and Nath 2014). Moreover, it has been recently observed that both HERV-K gag-pol and env RNA have significantly elevated expression in brains from ALS patients compared to controls (Li et al. 2015).
Expression of HERV-K in human neuronal cultures caused neuronal cytotoxicity, as observed by the decreased number of neurons and also the retraction of neurites in a dose dependent manner after the transfection of the entire HERV-K genome or of the HERV-K-env gene only. This suggested that intracellular HERV-K-Env protein could contribute to neurotoxicity. This has been confirmed by the CRISP/casp9 assay which has permitted HERV-K twofold increased expression through its LTR activation by the VP64 transcription factor (Li et al. 2015). HERV-K expression causes in vivo loss of the motor cortex volume in transgenic mice expressing the HERV-K-env gene in cortical neurons which is independent of the immune reactivity as measured with the ionized calcium-binding adapter molecule 1 (Iba-1) marker for microglia (Li et al. 2015). Behavioral analyses revealed that HERV-K-env transgenic mice traveled shorter distances, rested for longer periods and fell faster in a rotarod performance test displaying evidence of spasticity with increased clasping of the hind limbs. In addition to these motor dysfunctions, transgenic mice developed profound weakness of the limbs and spinal muscles including those for respiration resulting in 50% mortality by 10 months (Li et al. 2015).
Interestingly, HERV-K RT expression correlated with increased TDP-43 levels in neurons from ALS patients, suggesting that RT expression occurs in combination with other aberrant cellular processes characteristic of the disease (Buratti and Baralle 2009; Geser et al. 2009; Douville et al. 2011). Evidence for such a prion-like mechanism in ALS now involves the main misfolded proteins, SOD1 and TDP-43 (Polymenidou and Cleveland 2011). Recently, Li and al demonstrated that TDP-43 could activate HERV-K-env expression in human neuron, which is consistent with their observation that TDP-43 can bind to the region 726-CCCTCTCCC-734 (SEQ ID NO: 10) of HERV-K long terminal repeat (LTR) (Li et al. 2015). They also showed that endogenous TDP-43 silencing decreased HERV-K expression. These results have recently been complemented by showing that normal TDP-43 has no effect on HERV-K transcription in human astrocytes and neurons in vim), whereas TDP-43 has a binding site in the US region of HERV-K promoter. The latter binding is enhanced with inflammation, e.g. in presence of Tumor Necrosis Factor (TNF.alpha.), or with proteasome inhibition (Manghera, Ferguson-Parry, and Douville 2016). Interestingly, the same study showed that overexpression of aggregating forms of TDP-43 enhanced HERV-K viral protein expression and accumulation, when wild-type (normal) TDP-43 did not (Manghera, Ferguson-Parry, and Douville 2016). Moreover, despite evidence of enhanced stress granule and autophagic response in ALS cortical neurons, these cells failed to clear the excess HERV-K protein accumulation. Typical of most retroviral restriction factors, the TDP-43 promoter is likely to respond to interferon- and inflammation-associated transcription factors, as it contains binding sites for interferon regulatory factors (IRF1, IRF3) and nuclear factor-kappa B (NF.kappa.B) (Douville et al. 2011).
Taken together, these findings suggest that endogenous retroviral elements and HERV-K in particular are involved in the pathophysiology of ALS and could be the missing link between TDP43 and this proteinopathy (Alfahad and Nath 2013). HERV-K envelope protein expression within neurons of patients with ALS can therefore contribute to the neurodegeneration and disease pathogenesis.
To date, as a symptomatic treatment, Riluzole remains the only relatively effective drug and only extends the average survival of patients by 3-6 months (Hardiman, van den Berg, and Kieman 2011). Present treatment protocols are based on symptom management and on preservation of quality of life, provided in a multidisciplinary setting. The discovery of an efficient therapy remains a critical need for patients with this rapidly fatal disease (Hardiman, van den Berg, and Kiernan 2011).
Consequently, there remains an unmet need for effective therapeutic agents for treating ALS.